The present invention relates to a halogen-free prepreg formed of a resin composition having excellent flame retardancy and having good electric characteristics and heat resistance, for an insulating material, and a laminated board formed of the prepreg. The resin composition used in the present invention gives a cured product having good flame retardancy and electric characteristics and having excellent moisture absorption resistance and heat resistance and is therefore suitable for use in insulating materials. A laminated board formed of the above resin composition is halogen-free and phosphorus-free, satisfies the V-0 level of the UL standard generates neither dioxin nor phosphine during its combustion, and is therefore environmentally friendly.
Laminated boards formed of epoxy resins as a main component are widely used as printed wiring materials for electronic machines and equipment. For flame-retarding epoxy resins, there is employed a method of using brominated epoxy resins as basic components. In recent years, it has been found that intensely toxic brominated dibenzodioxin or dibenzofuran is generated during the combustion of bromine-containing flame retardants, and regulations to be imposed on the bromine-containing flame retardants are discussed in various ways from the aspect of so-called environmental problems. As a result, in the field of thermoplastic resins, phosphorus-containing flame retardants have come to be used in place of the bromine-containing flame retardants. In the field of printed wiring materials using epoxy resins as a base material, various flame retardants to be incorporated are studied and various proposals have been made for excluding brominated epoxy resins. Methods of using phosphorus-containing flame retardants constitute the main stream in proposals presently found. However, the phosphorus-containing flame retardants not only degrade printed wiring materials to a great extent in mechanical, chemical and electrical properties, but also inevitably generate toxic gases during the combustion thereof. Under the present circumstances, there is a considerable doubt about the use of the phosphorus-containing flame retardants. It is therefore strongly desired to develop a material having excellent flame retardancy without using any one of a bromine-containing flame retardant and a phosphorus-containing flame retardant.
Flame-retardant epoxy resin laminated materials that are now available are generally fabricated by incorporating an amine-containing curing agent to a brominated epoxy resin as a base resin to obtain a varnish, impregnating a glass substrate with the varnish, semi-curing the resin under heat to obtain a prepreg and forming the prepreg into a laminate. Few flame-retardant laminated materials overcome the above environmental problems.
It is an object of the present invention to provide a halogen-free and phosphorus-free prepreg having good flame retardancy and having excellent electric characteristics and heat resistance.
According to the present invention, there is provided a prepreg formed of a resin composition comprising, as essential components, a phenol compound (I) having a structural unit of the formula (1), 
wherein R1 is hydrogen or phenol, R2 is a hydrogen or methyl and n is an integer of 0 to 10, an epoxy resin (II) prepared by epoxidizing the same phenol compound as said phenol compound, a metal hydrate (III) and a molybdenum compound (IV), and a substrate which is impregnated with said resin composition or to which said resin composition is applied.
According to the present invention, there is also provided a laminated board formed of the above prepreg, for an insulating material.
For achieving the above object, the present inventors have made diligent studies and have found that a laminated material obtained from an epoxy resin composition containing an epoxy resin obtained from a specific phenol compound and a specific epoxy resin as a curing agent and further containing a metal hydrate and a molybdenum compound in combination has excellent flame retardancy and good electric characteristics and heat resistance. The present invention is accordingly achieved.
That is, the present invention is concerned with a prepreg formed of a resin composition comprising, as essential components, a phenol compound (I) having a structural unit of the formula (1), 
wherein R1 is hydrogen or phenol, R2 is a hydrogen or methyl and n is an integer of 0 to 10, an epoxy resin (II) prepared by epoxidizing said phenol compound, a metal hydrate (III) and a molybdenum compound (IV), and a substrate which is impregnated with said resin composition or to which said resin composition is applied. Further, the present invention is also concerned with a laminated board formed of the above prepreg, for an insulating material.
The phenol compound (I) used in the present invention is not specially limited so long as it is a compound having the structural unit of the formula (1). Typical examples of the phenol compound (I) include bisphenol F, phenol novolak, cresol novolak, tris(hydroxyphenyl)methane and an oligomer of tris(hydroxypheny)methane. These compounds may be used alone or in combination. Preferred are phenol novolak and an oligomer of tris(hydroxypheny)methane.
The amount of the phenol compound (I) is adjusted such that the molar ratio of phenolic hydroxyl groups to epoxy groups of the epoxy resin (II) is in the range of from 0.5 to 1.5, preferably from 0.8 to 1.2. When the above molar ratio is smaller than the above lower limit, undesirably in view of the object of the present invention, the resin composition shows very low heat resistance. When it exceeds the above upper limit, undesirably, the resin composition shows a very low humidity durability.
The epoxy resin (II) used in the present invention is not specially limited so long as it is a compound prepared by epoxidizing the same phenol compound as the phenol compound (I) having the structural unit of the formula (1). Typical examples of the epoxy resin (II) include glycidyl ethers of bisphenol F, phenol novolak, cresol novolak and tris(hydroxyphenyl)methane or derivatives of these compounds. These epoxy resins may be used alone or in combination. Preferred are glycidyl ethers of bisphenol F, phenol novolak and tris(hydroxyphenyl)methane or derivatives of these compounds.
The resin composition forming the prepreg in the present invention may contain other epoxy resin so long as the intended properties of the resin composition are not impaired. The above xe2x80x9cother epoxy resinxe2x80x9d can be selected from generally used epoxy resins without any special limitation. Typical examples of the other epoxy resin include glycidyl ethers of bisphenol A, bisphenol A novolak, biphenyl, polyfunctional phenol, naphthalene, alicyclic compounds, glycidyl amine resin, and glycidyl ester resins.
In the present invention, a known epoxy resin curing agent may be used in combination with the above phenol compound (I) so long as the intended properties are not impaired. The above epoxy resin curing agent can be selected from generally used epoxy resin curing agents without any special limitation. Typical examples of the epoxy resin curing agent include amine-containing compounds such as dicyandiamide, tetraethyldiaminodiphenylmethane and diaminodiphenylmethane, novolak compounds such as bisphenol A novolak, and acid anhydride compounds such as hexahydrophthalic acid anhydride. In the present invention, a curing promoter may be incorporated for adjusting the curing rate of the above resin composition as required. The curing promoter can be selected from curing promoters generally used for epoxy resins, without any special limitation. Typical examples of the curing promoter include imidazoles, derivatives thereof and tertiary amines.
The metal hydrate (III) used in the present invention is not specially limited so long as it is selected from hydrous compounds of alkali metals or alkaline earth metals. Examples of the metal hydrate (III) include aluminum hydroxide, magnesium hydroxide, a solid solution of magnesium hydroxide with zinc, nickel, calcium, aluminum or the like, and mixtures of these. These metal hydrates are those which are generally used as flame retardants for synthetic resins. The amount of the metal hydrate (III) per 100 parts by weight of the total of the phenol compound (I) and the epoxy resin (II) is 20 to 250 parts by weight, preferably 50 to 150 parts by weight. When the amount of the metal hydrate (III) is smaller than the above lower limit, there is not much effect on the improvement of flame retardancy. When it exceeds the above upper limit, the application performance of the resin composition to a substrate comes to be poor.
The molybdenum compound (IV) used in the present invention refers to a compound containing molybdenum, and it is not specially limited so long as it is selected from generally used molybdenum compounds. Examples of the molybdenum compound include molybdenum oxide, molybdenum borate, molybdenum silicate, molybdenum sulfide, zinc molybdate, calcium molybdate and ammonium molybdate. These molybdenum compounds may be used alone or in combination. Preferred is a material prepared by allowing calcium carbonate, zinc oxide or talc to support zinc molybdate (KEMGARD 911A,B,C, supplied by Sherwin-Williams). The amount of the molybdenum compound (IV) per 100 parts by weight of the total of the phenol compound (I) and the epoxy resin (II) is 0.1 to 20 parts by weight, preferably 0.5 to 10 parts by weight. When the amount of the molybdenum compound (IV) is smaller than the above lower limit, the flame retardancy is not much improved. When it exceeds the upper limit, the peel strength of a copper foil is low.
In the present invention, the resin composition may contain other flame retardant and a filler so long as the intended properties are not impaired. The above flame retardant and the above filler can be selected from those which are known and generally used.
Typical examples of the flame retardant include nitrogen-containing compounds such as melamine, benzoguanamine compound, powders of inorganic substances such as zinc borate and zinc stannate, and a silicone compound. Typical example of the filler include inorganic powders such as silica, mica, talc, glass short fibers, finely milled glass powders and hollow glass, and organic powders such as a silicone powder.
In the present invention, an organic solvent is used as required, while the organic solvent is not specially limited so long as it is compatible with the resin composition. Typical examples of the organic solvent include acetone, methyl ethyl ketone, methyl cellosolve, propylene glycol methyl ether and acetate thereof, toluene, xylene and dimethylformamide. These organic solvents may be used alone or in combination. When the impregnation of a substrate with the organic solvent is weighed, it is preferred to use solvents having boiling points of approximately 120 to 200xc2x0 C. in combination.
In the present invention, the resin composition may contain an ultraviolet absorbent, an antioxidant, a photopolymerization initiator and a fluorescence brightener so long as the intended properties are not impaired. These are not specially limited so long as they are known and can be generally used. Typical examples thereof include ultraviolet absorbents such as a benzotriazole-containing compound, antioxidants such as a hindered phenol-containing antioxidant and styrenated phenol, photopolymerization initiators such as a thioxanthone compound, and fluorescence brighteners such as a stilbene derivative.
The resin composition containing, as essential components, the phenol compound (I), the epoxy resin (II), the metal hydrate (III) and the molybdenum compound (IV) is applied to a substrate, or a substrate is impregnated with the above resin composition, and the resin composition is B-staged by heating, whereby the prepreg of the present invention is produced. The substrate used in the present invention can be selected from those substrates which are known and generally used in various laminated boards as electrically insulating substrates. Typical examples of materials for the substrate include inorganic fibers of E, D, S and Q glasses, organic fibers of polyimide, polyester and tetrafluoroethylene, and fibers of mixtures of these. The substrate includes a woven fabric, a non-woven fabric, a roving, a chopped strand mat and a surfacing mat if classified on the basis of its forms. The material and the form are properly selected depending upon use and performances of an intended laminated product. The above materials may be used alone or in combination, and the above forms may be employed alone or in combination, as required. Although not specially limited, the substrate generally has a thickness of approximately 0.03 to 0.5 mm. Further, it is preferred to surface-treat the substrate with a silane coupling agent or mechanically open the substrate, from the aspect of resistance against moisture absorption and heat. Concerning the amount of the resin composition adhering to the substrate, the prepreg after dried has a resin content of 20 to 90% by weight. After the substrate is impregnated with the resin composition, or the resin composition is applied to the substrate, the prepreg of the present invention is obtained by a method in which the resin composition is generally heated in a dryer having a temperature of 100 to 200xc2x0 C. for 1 to 30 minutes, to semi-cure (B-stage) the resin composition.
The laminated board of the present invention is obtained by laminate-forming the above prepreg of the present invention. Specifically, a plurality of prepreg sheets are stacked as required, a metal foil such as a copper or aluminum foil is placed on one surface, a metal foil is optionally placed on the other surface, and the so-structured set is formed into a laminate, whereby a laminated board is obtained. The metal foil is not specially limited so long as they are those which are used in the field of general printed wiring board. The above laminate-formation is carried out under conditions employed for forming general laminated boards and multi-layered boards used as insulation materials. For example, the laminate-formation is carried out with a multi-platen press, a multi-platen vacuum press, a continuous-forming machine or an autoclave forming machine at a temperature of 100 to 250xc2x0 C. under a pressure of 2 to 100 kg/cm2 for a heating time period of 0.03 to 3 hours. Further, the prepreg of the present invention and a separately prepared printed wiring board for an internal layer may be combined and formed together into a laminate, whereby a multi-layered board can be obtained.